Contrary to a popular belief, dead dinosaurs do NOT turn into oil. Since almost all oil comes from rocks that were formed underwater, floating ocean life (tiny, tiny creatures known as diatoms, foraminifera, and radiolaria - all just as small as a grain of sand) that settle to the bottom of the sea is what eventually turns into oil. How could something so small give us so much oil? Well, it takes many thousands of years (and millions of dead critters) to form thick deposits of organic-rich sludge at the bottom of the ocean. But how does this sludge turn into oil? For the answer, see "maturation".